Orb-Web Weaving Spiders: Strategies of Survival

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1 Jaime Chastain April 6, 2013 BSPM Orb-Web Weaving Spiders: Strategies of Survival Biologists believe there are thousands of spiders that remain undiscovered in our world (Explorit). One way to categorize the spiders we know of is by the type of web they spin. Some spiders do not spin webs at all; rather, they hide in trap doors underground. Nonetheless, all spiders produce silk and even trap-door spiders use their silk to line their burrow. Of the web-spinning spiders, many web types are commonly found in nature: the sheet web, orb web, triangle web, cobweb and mesh web, funnel web, tubular web, and tent web (Biokids.com). This alone displays the diversity and complexity of a spider. There are many different behaviors and strategies exhibited by spiders, making it a study of great wonder. There is still a lot to be learned about spiders, but great steps have been made toward understanding the behaviors of the orb-web weaving spiders, anatomical significances and ecological interactions of spiders. Avoidance of Sticking to own Web: Webs are capable of catching large sized prey, yet spiders do not stick to their own web. Spiders use a few different mechanisms to avoid sticking to their own web. The easiest tactic to identify is leg movement; spiders move carefully and avoid their sticky lines of silk when moving on their web, which are the concentric rings of the web. The radial lines are not as sticky and spiders move along these lines as much as possible. Less apparent, though, is various bundles of branched setae on their legs. The setae

2 reduce the contact surface area of the leg with the web, decreasing the force required to pull the leg off. If the leg is pulled off of the sticky lines with proper orientation, the gluedrops slide right off of the setae. The branched quality of the setae prevents glue droplets from reaching the surface of the leg, which would have much stronger adhesion (Eberhard 2012). Another mechanism spiders are thought use to avoid their web is spreading chemicals from their mouthparts onto their legs. Eberhard found that there are three chemicals present from mouthparts: n-dodecane, n-tridecane, and n-tetradecane. In addition to these, Kropf and his team of researchers found that molecules already present on the legs contribute as well. They tested for the presence of an organic molecule on spider legs by washing the legs with distilled water and an organic compound, carbon disulphide (CS2). They compared the adhesion of these legs with untreated legs and found that the carbon disulphide treated legs stuck twice as strongly to the sticky silk lines than the water-washed and untreated legs. The water did not show a significant change in adhesion from untreated legs. This showed that the product of a reaction between the molecules was no longer functional in keeping the spider from sticking to its web. It is unclear whether the molecule on the leg of the spider is fatty or chemical, but its presence is certain. Spiders show both behavioral and evolutionary traits that help them to not stick to their webs. Pollen Feeding: When young spiders first hatch, they often have a hard time finding food since they are small nymphs and do not have experience in capturing prey. Chances of survival are low if the spider cannot find a food source soon enough after hatching. Once young

3 spiders build a web, pollen and spores get caught in the web, which provide a temporary diet, which may keep them alive until they molt into an adult size. The energy provided by the pollen provides the young spiders with enough energy to build upon their web and construct more. They eat the silk to recycle its energetic value, but the energy it provides when recycled is less than it took to spin the web, resulting in loss of energetic value. Smith and Mommsen tested the effects of eating pollen on web-spinning behavior in comparison to insect-fed, spore-fed and starved young spiders. They found that the pollen-fed spiders had sufficient energy and took down and re-spun webs more often than starved, insect-fed or spore-fed spiders. However, only the insect-fed spiders could molt, implying a nutrient in insects is required to commence a molt. Spiders can change their diet temporarily in order to survive in less than ideal conditions, but insects are necessary for a spider to grow into its adult form. Importance of Various Silks: Spiders use several types of silk to spin webs. There are seven known spigots that produce different types of silk compounds. Not all are present in every spider, but at least two or more of the known seven are. Silks are used for many different reasons, some being construction of egg sacs, sticky capture-threads of the web, and radial lines on the web for movement and support of the sticky threads. In a recent analysis done by William G. Eberhard (2010), he noticed the location of different spigots on spinnerets of the spider gives insight to the function of the silk glands associated with the spigot. Dragline spigots are anterior, piriform and cribellum spigots are both anterior and planar, and aciniform are posterior. The location effects how the line is attached to the substrate.

4 Eberhard proposes he sticky lines for egg sacs are not from the piriform gland, as initially thought. Rather, they are attached to draglines and the sticky thread comes from aciniform glands that are laid with liquid that increases its adhesive properties. Major ampullate spigots are laid downstream from others, supporting an upstream-downstream hypothesis of the functionality of anterior, posterior, and planar spigot arrangements. The major ampullate spigots being downstream others raises the possibility that they initiate lines form other glands. The functionality and behavioral significance of spigot placement has not been extensively studied, however Eberhard observed spiders raising and lowering posterior spigots asymmetrically when wrapping prey. They also use their legs to orient the silk lines when laying the threads. Jaromir Hajer found that the spider Harpactea rubicunda uses three of the seven known silk glands: major ampullate silk glands, piriform and pseudaciniform. From the pseudoaciniform spigot, web-spinning silks are produced as nano-fibril bundles that the spider overlaps and interweaves while spinning, which adds strength to the web. This behavior is an example of how spiders can orient their silk lines with their legs to increase functionality of the web. In a later study by Todd A. Blackledge, the functionality of pseudo-orb webs is compared to that of true orb webs. Pseudo-orb webs have a different silk composition than true orb webs due through convergent evolution. Both silks have MaSP1 protein, but only the true-orbs have MaSP2 proteins, which increases extensibility and compliance of the silk by disrupting intermolecular crystalline structures. This results in pseudo-orb webs having stronger major ampullate silks, yet they are stiffer than true-orb web major ampullate silks. The pseudo-orb webs are less efficient in capturing large, fast-moving

5 prey since it does not dissipate the kinetic energy as well. The kinetic energy is delivered by the prey to the web when it is caught, and the more extensibility a silk line has, the easier it is to dissipate the energy. The feeding behaviors between pseudo-orb weavers and true-orb weavers vary, due to the difference in their silk composition. Pseudo-orbs are limited to smaller prey; true-orbs can capture more variety of prey. A structural component that has evolved in true orb-web spiders is the presence of sticky glue drops on the silk, as shown by Agnarsson. The stickiness of the glue corresponds linearly to the strength of prey capture. The adhesiveness of the glue droplets is always less than the force required to break the silk line. Therefore, the tension in the silk lines is more important to successful prey capture than the stickiness and composition of the glue drops. Depositing glue droplets on silk lines that have higher tension results in more efficient prey-capture since the prey repeatedly sticks to the line; repeatedly sticking to the web increases the time it takes for the prey to escape, thus giving the spider more time to reach the prey. Methodical placement of glue droplets on the web can increase the spider s chances of successful prey-capture. Adjustments Made in Limited Spaces: Spiders show adaptability in web construction when limited to smaller spaces. Barrantes found that there are at least seven independent traits a spider can adjust, including radius of the web, frame construction, hub(center) construction, the construction of the first sticky spiral loop and the rest of the sticky spiral, termination of sticky spiral threads, and the number of radial lines. The size of these areas of the web was reduced as space was limited as well as the number of spirals and radial lines in the

6 web, supported by Barrantes hypothesis of continuity. Spiders can evaluate their environment and predetermine necessary structural changes to their web design to conform to smaller areas. Some aspects, such as web symmetry, did not show a continuous decrease as space became more restricted. The theory on continuity of adjustments in webs as construction areas decrease has significant support, but there is much room to expand and test this theory. Spiders are able to test environments they are unsure of when relocating by spinning webs with less silk at first. If a spider is not certain of a location and it spins a large web and later finds that the new site has low prey density, they suffer a large energetic cost. The spider lost energy to the construction of the web and did not capture sufficient prey to increase their energetic stores. Spiders display strategic behaviors to balance the cost-benefit tradeoff of relocating versus web size. Foraging is the main strategy implemented by spiders to scout out new areas. Kensuke Nakata found that spiders who forage frequently are more certain of the new web location and the relative prey density. They do not relocate as frequently, but more silk can be used in their new web initially since they have more certainty in successful prey-capture at the new site. Spiders that do not forage as frequently relocate more often. They use less silk initially when constructing new webs to test the prey density of the new location; if it proves adequate, the spider build upon the web. Since they are less certain of the new site than spiders that forage often, they cannot expend a lot of energy at the new site until they are more certain of a high prey density. Spiders can adjust web-size based on how certain they are of successful prey-capture at the new site. They methodically balance the energy spent to what they expect to get back from their diet.

7 Mesh Width and Behavior: The distance between sticky spiral capture rings affects the prey-capture effectiveness of the web. There is a tradeoff between visibility of the web and ability to catch prey. Webs that have smaller mesh widths are more visible but they absorb more kinetic energy. Blackledge compared the capture rates of webs with different mesh widths on four different insects (deerflies, large grasshoppers, small grasshoppers, and hanging flies). He found that for deerflies and large grasshoppers, there was a decrease in retention time with larger mesh widths. The other two insects did not appear to be affected by mesh width. Mesh width may be determined with a certain prey in mind, local to the spider s habitat. This behavioral strategy allows the spider to determine how close together the spiral silks can be to successfully catch large prey and remain unseen. Spiders need large meals occasionally for reproductive success, but will eat almost any prey caught in their web. Blackledge also found that well-fed spiders made webs with smaller mesh widths. This is probably because of their success at the location of their web; they have high prey density, so visibility is not as costly. Although the data did not all agree with the hypothesis, there were significant results for two of the four insects, which is great groundwork for future experimentation on mesh width and behavioral strategy in web construction. Kleptoparasitism: Kleptoparasitism is a behavior exhibited by some spiders; they linger on the outer edges of webs and wait for opportunity to take energy sources from the host spider. Two

8 energy sources they have been observed to take are captured prey and the silk of the web itself. The most common form of kleptoparasitism is prey stealing. Rypsta observed the behaviors of the kleptoparasitic spiders on orb-web weaving host spiders (Nephilia Clavipes). She found that spiders were more likely to relocate their webs if the spider is molting, laying eggs, or if there is frequent web destruction. Occasionally they will abandon their old web without recycling the energetic benefits of the silk and build a new home elsewhere. This has been linked to presence of parasites on their old web or to avoid predation. Parasitic spiders typically go for the smaller prey captured. The host spider did not show any awareness of the parasitic spiders lingering in the web. So, when the parasites steal the host s prey, the host thinks capture rates are low due to low density. More often though, as found by Tso, the host spider will not find the need to relocate since most of the biomass in their diet comes from larger prey. Small prey are comparable to small snacks in between meals and do not have a large impact on overall survival rate if the web catches large prey periodically. There has not been sufficient research to fully understand the dynamics of this relationship and the behavioral consequences. Some areas to further be explored are the awareness of the host spider of the parasitic spiders and true causes of abandoning old webs in host spiders. Another type of kleptoparasitism is silk stealing. Some parasitic spiders sit at the edges of a host spider s web and eat spiral silk lines. Silk-stealing seems to be more harmful to the host spider than prey-stealing, since silk is always present and it does not require energy expenditure to steal if the parasites eat from the outside in. Tso looked at the parasitic strategy of silk stealing in Argyrodes lanyuensis and whether or not it had a high cost to the host spider, Nephila maculata. He found a negative relationship between

9 body length of the host spider and the amount of silk stolen, revealing small spiders are easier to steal silk from. This makes sense, since smaller spiders do not have as large of energy stores as an adult spider and may not be as experienced. He also found a negative correlation between the diameter of the silk and the amount of silk lost. Granted, web reduction based on spiral silks is a very hard measurement to make, but parasitic spiders tended to prefer thin silk. This is a subject that could be extensively studied but has not been so far. There are many possibilities as to why thin silk might be preferred: it s less noticeable, it is thinner on the outer spiral silks anyway, it might benefit the host spider in some way, or maybe it is easier for young spiders to eat. Young parasitic spiders rely on silk-strealing to survive. It is unclear if this relationship is truly parasitic or if the host spider is unaffected by the presence of silk-stealers. Nocturnal spiders: Nocturnal spiders have developed a strategy to increase their chances of prey capture. Since they are active at night, they orient their webs near light. Many nocturnal insects flock toward light sources. Porch lights and street lights show tons of insects swirling around it at nighttime. Heiling tested the attractiveness of light in female nocturnal orb-web spiders (Larinioides sclopetarius). She found that females of the species always built their webs closer to a light source. The spider exploits the attraction to light of other insects by building her web closer to the light. The chances of an insect flying into her web are much higher than if she built her web in the dark. Although unseen in unlit areas, the web is in a location insects tend to avoid while venturing

10 through the night. This behavior is advantageous to the spider and shows practical decision-making when it comes to web location. Conclusion Orb web spiders exhibit very complex behaviors when designing webs, dealing with pests, and finding enough food for survival. Webs can be adjusted in many different ways. They can be reduced in span or in inner-web structure ratios. Several silks serve unique purposes in the construction of the web. The spider is able to chose the proper silk and place it carefully by moving its spinnerets and adjusting the orientation of the silk line using its legs. Silks have different composition between spider species, depending on the type of silk spigots and glands present it varies from species to species. Pseudo-orb web spiders do not show as much extensibility in their silk lines, nor glue droplets; the extensibility of true orb webs allows kinetic energy from the prey to easily dissipate through the silks, rather than flinging the prey off the web when the silks return the applied force of impact. A spider can assess its situation and decide whether a more visible, yet more effective, web is worth the extra energy. Mesh width is a continuous trait of spider webs and can be adjusted to many different widths. As mesh width changes, there is an inverse relationship between visibility and effectiveness. Besides adjustments to the web, spiders show behavioral responses to stress factors in their environment. Well-lit areas are preferred for web construction in nocturnal orb-web spiders since they can exploit insect attraction to light. Parasitic spiders on the web of host orb-web spiders do not seem to affect the host, even though they steal small prey.

11 Silk stealing seemingly has more of a negative impact, however the impact has not been quantified or fully determined. The silk does benefit the parasitic spiders, since the young spiders can obtain energy through eating the host s silk. Young spiders have also been known to eat pollen grains caught in their webs, which increases their chances of survival. As time progresses and more spider species are discovered, our understanding on the behaviors of web-spinning spiders will increase. The behaviors are very complex and are unique to each species of spider as well as the specific environmental conditions each species experiences in their habitat. As more research is done, the complexity of spiders becomes more apparent. References: Agnarsson, I. and T.A. Blackledge. Can a spider web be too sticky? Tensile mechanics constrains the evolution of capture spiral stickiness in orb-weaving spiders. Journal of Zoology 278 (2009): Print. Barrantes, Gilbert and William G. Eberhard. Extreme Behavioral Adjustments by an Orb-Web Spider to Restricted Spaces. Ethology 118 (2012): Print. Blackledge, Todd A., et al. Biomaterial evolution parallels behavioral innovation in the origin of orb-like spider webs. Scientific Reports (2012): 1-6. Print. Blackledge, Todd A. and Jacquelyn M. Zevenbergen. Mesh Width Influences Prey Retention in Spider Orb Webs. Ethology 112 (2006): Print. Briceno, R.D. and W.G. Eberhard. Spiders avoid sticking to their webs: clever leg movements, branched drip-tip setae, and anti-adhesive surfaces. Naturewissenschaften 99 (2012): Print. Eberhard, William G. Possible functional significance of spigot placement on the spinnerets of spiders. BioOne 38.3 (2010): Print. Eberhard, William G. and William T. Wcislo. Grade Changes in Brain-Body Allometry: Morphological and Behavioral Correlates of Brain Size in Miniature Spiders, Insects, and Other Invertebrates. Advances in Insect Physiology 40 (2011): Print. Eberhard, William G. and Thomas Hesselberg. Cues that Spiders (Arneae: Araneidae, Tetragnathidae) Use to Build Orbs: Lapses in Attention to One set of Cues because of Dissonance with Others? Ethology 118 (2012): Print. Hajer, Jaromir, et al. Silk Fibers and Silk-Producing Organs of Harpactea rubicunda (C.L. Koch 1838) (Araneae, Dysderidae). Microscopy Research and Technique 76 (2013): Print.

12 Heiling, Astrid M. Why do nocturnal orb-web spiders (Araneidae) search for light? Behavioral Ecology and Sociobiology 46 (1999): Print. Kropf, Christian, et al. An organic coating keeps orb-weaving spiders (Araneae, Araneoidea, Araneidae) from sticking to their own capture threads. Journal of Zoology Systematics and Evolutionary Research 50.1 (2012): Print. Nakata, Kensuke and Atushi Ushimaru. Difference in Web Construction Behavior at Newly Occupied Web Sites Between Two Cyclosa species. Ethology 110 (2004): Print. National Science Foundation. BioKids: Kids Inquiry of Diverse Species. Interagency Education Research Initiative and University of Michigan and Detroit Public Schools Web. 12 March Rypstra, Ann L. The Effect of Kleptoparasitism on Prey Consumption and Web Relocation in a Peruvian Population of the Spider Nephila clavipes. Oikos 37.2 (1981): Print. Smith, Risa B. and Thomas P. Mommsen. Pollen Feeding in an Orb-Weaving Spider. Science AAAS (1984): Print. Spider Facts. Explorit Science Center. The Davis Enterprise, n.d. Web. 10 April Tso, I-Min and Lucia Liu Severinghaus. Silk Stealing by Argyrodes lanyuensis (Araneae: Theridiidae): a unique form of kleptoparasitism. Animal Behavior 56.ar (1998): Print.